Carolina Panthers defensive end Julius Peppers, the top candidate for rookie of the year honors, faces a four-game suspension after a test was deemed positive for a banned substance, league sources confirmed Thursday.

The potential suspension was first reported by the Charlotte Observer in its Thursday editions.

Two independent sources told ESPN.com that Peppers, the NFL's sacks leader through 10 weeks of the season, tested positive for either ephedrine or a derivative of the stimulant ephedra.

The substance, typically absorbed through supplements, is on the league's list of banned substances. The NFL began testing for it this summer.

A four-game suspension is automatic when a player tests positive for a banned substance or for steroids. The automatic suspension differs from the NFL's "tiered" sanctions for players who test positive for drugs. Neither source would confirm when the positive test took place but it is believed to have been after training camp.

The league will conduct a second screening, likely by the end of the week. Peppers has the right to appeal the findings even if that test registers a positive result.

If suspended, Peppers would become the second Carolina defensive lineman to be so sanctioned. NFL officials on Nov. 4 suspended tackle Brentson Buckner for four games. Sources said Buckner's suspension also was for a banned stimulant.

Citing confidentiality guidelines, the NFL does not announce the substance for which players test positive, generically only saying that a player violated the substance and steroid abuse policy. But last week Kansas City Chiefs linebacker Lew Bush was suspended, and sources confirmed he tested positive for ephedrine and another banned stimulant, Ma Huang.

Peppers, 22, was the second overall player selected in the 2002 draft out of North Carolina. He signed a seven-year contract that could be worth up to $62 million, and which included a $9.1 million signing bonus. For that, the Panthers have seen Peppers succeed immediately with 10 sacks from his left-end position.

A four-game suspension would cost Peppers $235,294 in base salary, four-seventeenths of his scheduled $1 million base salary.